1. Technical Field
This invention relates to optical spectrum analyzers and monochromators of the kind which use a diffraction grating. The invention is especially applicable to optical spectrum analyzers in which the light beam to be analyzed is applied to the diffraction grating more than once so as to obtain improved resolution, and to monocchromators for use therein.
2. Background Art
The invention is concerned especially with an optical spectrum analyzer of the kind disclosed in U.S. Pat. No. 5,986,785 issued March 1999 naming H. Lefcvre et al. as inventors. Lefevre et al. disclosed an optical spectrum analyzer comprising a diffraction grating and a dihedral reflector. The input light beam for analysis is received via an input/output port, collimated, and passed through a polarization splitter which splits the light beam into two linearly-polarized components having their respective directions of linear polarization mutually perpendicular. The transmitted beam is passed through a waveplate which rotates its direction of polarization through 90 degrees so that the two beam components leaving the beam splitter are directed onto the diffraction grating with their directions of polarization parallel to each other and perpendicular to the groove direction of the diffraction rating.
Following diffraction, the light beam components are directed to the dihedral reflector which reflects them back to the diffraction grating. Following diffraction a second time, the light beam components are returned to the polarization beam splitter which recombines them and passes the recombined light beam through the collimator in the opposite direction to focus it and direct it to the input/output port. In traversing the diffraction grating and dihedral reflector the light beam components both follow exactly the same path, but in opposite directions.
A disadvantage of their design arises from the fact that the light beam components are recombined and leave via the same exit port. Specifications for optical spectrum analyzers require the back-reflection caused by the analyzer to be below certain levels so as not to affect the equipment which is being tested. Consequently, an optical circulator can be used in the approach of Lefevre et al. to separate the input light beam from the output light beam and avoid back reflection. The insertion loss and isolation of the circulator vary with wavelength and imperfections in the circulator introduce cross-talk, i.e. coupling of energy from the input light beam directly to the output light beam within the circulator.
Instead of a circulator, Lefevre et al. could use a coupler to separate the input and output light beams and an isolator to reduce optical back reflection significantly Both the insertion loss and the isolation capability of such an isolator usually are wavelength dependent. Moreover, the coupler would introduce insertion loss of at least 6 dB, e.g. 3 dB in each direction for an ideal 3 dB (50/50) coupler. Also, directivity of the coupler introduces cross-talk between unfiltered input and output ends.
In general, the use of components such as couplers, circulators and isolators is accompanied by an inevitable inherent wavelength-dependent polarization-dependent loss, that cannot readily be compensated for, or taken into account.
With the increasing use of Dense Wavelength Division Multiplexing (DWDM), optical spectrum analyzers may be used to scan as many as 128 wavelengths. In view of this level of direct cross-talk, the optical xe2x80x9cnoise floorxe2x80x9d existing at the detection/receiver will increase proportionally with the number of channels, whereas the signal strength of each individual channel is fixed. This degrades optical signal-to-noise ratio (OSNR) of the instrument.
A further disadvantage arises from the fact that the separation of the input beam into constituent, orthogonal polarization states occurs within the monochromator section of their design, where the light beams are propagating in free space. This introduces a complication in the optical design and the choice of components, since the beam size may be limited by the clear aperture of the polarization beam splitter, which for cost and availability reasons should be kept as small as possible. On the other hand, the maximum spectral resolution is obtained when the largest possible number of grating grooves are illuminated. The addition of a beam expander (e.g. with anamorphic prisms) to avoid this problem would be unsatisfactory because it would be expensive and unwieldy.
It is noted that Lefevre et al. apparently recognised that the need for an optical circulator or 3-dB coupler could be avoided by placing a separate output fiber immediately adjacent to the input fiber. However, this modification would not entirely solve the problem of significant back-reflection into the input fiber and would not reduce OSNR degradation caused by back scattering, whereby light scattered by components within the monochromator is received by the output fiber.
It is desirable to avoid, or at least reduce, any undue wavelength-dependent polarization-dependent loss in this measurement instrument.
According to Lefevre et al., their optical spectrum analyzer is polarization insensitive. in practice, however, there is a wavelength-dependent loss resulting from polarization dependence of components used in their design, particularly the waveplate. This waveplate exhibits a xcex/2 retardance, resulting In a 90-degree rotation of the linear polarization, for a particular wavelength. As the wavelength of the incident light beam is tuned away from that wavelength, the angle of rotation provided by the waveplate will vary. Consequently, that beam component not having its linear state of polarization (SOP) perpendicular to the grooves of the grating will suffer increased attenuation as compared with the component which has its linear SOP perpendicular to the grooves.
Although this wavelength-dependent loss can be compensated in the firmware of the analyzer, it results in a limitation in the ultimately attainable instrumental sensitivity.
The present invention seeks to avoid or at least mitigate the afore-mentioned disadvantage.
According to one aspect of the present invention, an optical spectrum analyzer characterized by a diffraction grating (DG), input means (PDM, FP1/1, FP1/2) comprising means (PDM) for decomposing an input light beam to provide first and second light beams (LR, LT) each having a linear state of polarization corresponding to a respective one of two mutually-perpendicular linear states of polarization of the input light beam, and times (FP1/1, FP1/2) for directing the first and second light beams onto the diffraction grating (DG), and output means for directing the first and second lights beams after diffraction to two output ports (FP2/1, FP2/2) such that each port receives substantially exclusively a respective one of the light beams at or about a selected wavelength after diffraction by the diffraction grating, the arrangement being such that, at any particular wavelength within an operating band of the analyzer, the state of polarization of each of the first and second light beams remains substantially unchanged with respect to time.
Each of the first and second light beams may be incident upon the grating with its linear of polarization having any prescribed angle with respect to a corresponding plane of distraction.
Preferred embodiments of the invention further comprise means for effecting wavelength-independent rotation of one or both of the linear states of polarization of the first and second light beams so that the two linear states of polarization are aligned parallel to each other, and the means for directing the first and second light beams onto the diffraction rating does so with the linear states of polarization of the first and second light beams, respectively perpendicular to grooves of the diffraction grating.
The polarization decomposing means may comprise a polarization beam splitter coupled to the monochromator section by a pair of polarization maintaining fibers. One or both of the polarization maintaining fibers may be twisted to provide a required rotation of the state of polarization of the light beam passing therethrough.
According to a second aspect of the invention, there is provided a monochromator comprising a diffraction grating, means for directing first and second light beams of originally mutually-perpendicular linear states of polarization onto the diffraction grating, and two output ports each for receiving, substantially exclusively, a respective one of the linearly-polarized light beams at or about a selected wavelength after diffraction by the diffraction grating, the arrangement being much that, at a particular wavelength, the state of polarization of each of the first and second light beams is preserved.
Preferably, each of the two linearly-polarized light beams is incident upon the diffraction grating with its state of linear polarization parallel to a plane of diffraction of the diffraction grating (i.e. perpendicular to groove direction).
The monochromator may further comprise reflector means for reflecting the light beams leaving the diffraction grating after diffraction a first time so as to return them to the diffraction grating again with the same state of polarization and at a position displaced laterally from the position at which the light beams were first incident thereupon the two output ports receiving respective ones of the linearly-polarized light beams after diffraction a second time.
In this specification, the term xe2x80x9cgroovexe2x80x9d embraces both the physical grooves In a ruled diffraction grating and their functional equivalent in, for example, a holographic grating.
Various features, advantages and objects of the invention will become apparent from the following description of a preferred embodiment which is describe by way of example only with reference, to the accompanying drawings.